Boron Isotopes in Benthic Foraminifera by MC-ICPMS: Unlocking the Ocean's Carbon Cycle

The cause of glacial-interglacial CO2 cycles has been described as the "holy grail" of climate science. All models currently proposed invoke changes in deep ocean carbon storage, but the mechanisms by which this took place remain unclear. Proxies for two components of the ocean carbonate system would allow us to fully reconstruct ocean carbonate equilibria and trace the spatial and temporal pattern of glacial carbon storage, providing valuable constraints on the causal mechanisms of atmospheric CO2 change. The theory behind the boron isotope pH proxy is well understood, but its reliability has been questioned, primarily due to uncertainty in the fractionation factor between boron species in seawater, and analytical difficulties associated with negative thermal ionisation (NTIMS) measurements. We have developed a new technique for boron isotopic analysis by multicollector inductively coupled plasma mass spectrometry (MC- ICPMS), which overcomes many of the problems associated with NTIMS measurements. Our method is precise (better than 0.25%, or ~0.02 pH units, on full procedural replicates at 95% confidence), rapid (allowing duplicate measurement of 10-20 samples per analytical session), and has small sample size requirements of ~10 ng boron (~0.5 mg foraminiferal tests). As MC-ICPMS analysis requires separation of boron prior to measurement, any bias between samples and standards with different matrices is also removed. Recent experimental work has also improved uncertainty in the isotopic fractionation factor (now measured at 1.0272 ±0.0006 [1]), providing a powerful independent means to test the behaviour of the foram-based δ11B proxy, and its ability to provide absolute pH values. We have measured δ11B in several species of benthic foraminifera from a range of core-top samples. In contrast to previous studies, we find a very close match between foraminiferal δ11B values and the δ11B of seawater B(OH)4- - predicted using the recently determined fractionation factor of [1] - allowing a ready determination of deep water pH. Here we combine this with measurements of B/Ca ratios, a proxy for carbonate ion saturation [2], to determine all components of the ocean's carbonate system. We will present results from several key locations in order to gain insights into the mechanisms responsible for glacial-interglacial pCO2 change. [1] K. Klochko, A. J. Kaufman, W. Yao, R. H. Byrne, and J. A. Tossell (2006), Earth and Planetary Science Letters, 248, 261-270. [2] J. Yu and H. Elderfield (2007), Earth and Planetary Science Letters, 258, 73-86.